Graphite thermal neutron scattering cross section calculations including coherent 1-phonon effects

Graphite serves as the moderator and reflector for the very high temperature (VHTR) concepts that are currently under study as part of the proposed Generation IV reactors. The VHTR is a "thermal" reactor where, to propagate the chain reaction, neutrons born in fission slow-down and thermalize in the graphite moderator prior to initiating a new fission reaction. Consequently, accurate quantification of the thermalization phenomenon using thermal scattering cross sections enhances the ability to predict the thermal neutron spectrum in the core. For graphite, which is characterized by a strong coherent scattering behaviour, deviations have been observed between thermal neutron scattering cross sections that are calculated using the incoherent approximation and total cross section values measured using pyrolytic graphite samples. These deviations are remedied by accounting for the coherent 1-phonon component of the cross section. However, discrepancies remain with total cross section values that are measured using reactor grade graphite samples. In this case, the discrepancies are attributed to differences between the actual structure of reactor grade graphite and the structure assumed in the lattice dynamics model that is implemented in typical cross section calculations.